The objective of this study is to develop a system for continuously monitoring cardiac output by the use of Doppler ultrasound from the esophagus. The principle of operation is that blood velocity at the measurement site, the transverse aorta, will be proportional to the blood flow. It has already been demonstrated that continuous wave Doppler can accurately assess stroke volume in man. Recent studies by others have shown that flow velocities can be detected by pulsed Doppler sensors from the esophagus. Experiments are planned to quantify the requirements for an esophageal probe design and placement technique so as to be able to continuously monitor such flow. We wish to accomplish several specific objectives: 1) development of a continuous wave esophageal transducer system; 2) testing of this system in animals under conditions similar to clinical situations to verify operation and refinement of placement technique; 3) perform any necessary refinements of the transducer and system; and 4) evaluate the system in 20 human subjects in the intensive care unit and operating room. The objective of the clincal evaluations are to further refine the design of the system, to assess the feasibility o the technique in clinical settings, and to provide the basis for a more extensive evaluation to assess the efficacy of the technique and to develop applications to patient care.